Advertising in online mediums have some similarities with traditional print-based advertising. For example, it is usually desirable to target the advertisement to the intended or expected audience, because advertisers generally prefer to invest in advertising if it reaches many potential customers, as opposed to persons who have little or no interest in the product or service being advertised. In print media, this targeting could be based on the general knowledge about who the readers are, or market investigations can be performed on the readers to more accurately determine their interests.
In the online environment, the users likewise have particular interests and disinterests, and publishers sometimes try to match the advertisement displayed on a page to the particular viewer as well as possible. However, much of the information required to understand who the viewers are may not be known or readily available and thus the advertisement targeting may be based on little or no relevant information.